mktag tests: run "fsck" after creating "mytag"

mktag tests: run "fsck" after creating "mytag"

Change the last test in the file to run an "fsck --strict" after
creating the tag at the end.

We're just doing this for good measure to check that fsck behaves as
expected now that there's finally a reference for our valid tag. Other
tests going to be checking this elsewhere, but it's nice to cover all
the edge cases in this test to make it as self-contained as possible.

Signed-off-by: Ævar Arnfjörð Bjarmason <redacted>
Signed-off-by: Junio C Hamano <redacted>